Technological Field
The present technology relates to memory devices based on phase change materials, and methods for manufacturing and operating such devices.
Description of Related Art
In a phase change memory (PCM), each memory cell includes a phase change memory element. The phase change memory element can be caused to change phase between a crystalline state and an amorphous state. The amorphous state is characterized by higher electrical resistivity than the crystalline state. In operation of the phase change memory element, an electrical current pulse passed through the phase change memory cell can set or reset the resistivity phase of the phase change memory element. To reset the memory element into the amorphous phase, an electrical current pulse with a large magnitude for a short time period can be used to heat up an active region of the memory element to a melting temperature, and then cool quickly causing it to solidify in the amorphous phase. To set the memory element into the crystalline phase, an electrical current pulse with a medium magnitude, which causes it to heat up to a crystallization transition temperature, and a longer cooling time period can be used allowing the active region to solidify in a crystalline phase. To read the state of the memory element, a small voltage is applied to the selected cell and the resulting electrical current is sensed.
To achieve low power operation, the magnitude of the current needed for reset can be reduced by reducing the size of the phase change material element in the cell and/or the contact area between electrodes and the phase change material, such that higher current densities are achieved with small absolute current values through the phase change material element. As shown in FIG. 1A, a conventional “mushroom-type” memory cell 100 has a reduced contact area between first electrode 111 and phase change memory element 113. First electrode 111 extends through dielectric 112, phase change memory element 113 comprises a body of phase change material, and second electrode 114 resides on the memory element 113. First electrode 111 is coupled to a terminal of an access device (not shown) such as a diode or transistor, while second electrode 114 is coupled to a bit line and can be part of the bit line (now shown). First electrode 111 has a width less than the width of second electrode 114 and memory element 113, establishing a small contact area between the body of phase change material and first electrode 111 and a relatively larger contact area between the body of phase change material and second electrode 114, so that higher current densities are achieved with small absolute current values through memory element 113. Because of this smaller contact area at the first electrode 111, the current density increases in the region adjacent first electrode 111, resulting in the active region 115 having a “mushroom” shape as shown in FIG. 1A. FIG. 1B is a low angle annular dark field scanning transmission electron microscopy (LAADF-STEM) image of a cross-section of a mushroom-type memory cell comprising Ge2Sb2Te5 in the reset state. As seen in the FIG. 1B, a bulk amorphous mushroom-shaped region 116 is formed over the bottom electrode and surrounded by large crystalline grains.
For smaller width of electrodes, smaller currents are required for a reset operation. However, forming electrodes of sublithographic feature size involves complicated manufacturing processes, thereby increasing manufacturing costs. Moreover, the electrical and mechanical reliability issues increase with reducing the contact area.
It is desirable to provide memory devices having small reset current yet maintaining electrical and mechanical reliability.